Use event loop for data transfer

HTTPFileServer is not as Qt friendly as we want it to be: it uses a loop that blocks signal processing and makes big files cause a crash on Windows.

**What is *not* a solution:** flush(), waitForBytesWritten(), QCoreApplication::processEvents()
**Solution:** signals and slots, as most of QTcpSockets' functionality is implemented as those.

_See http://doc.qt.io/qt-5/qabstractsocket.html#flush and http://doc.qt.io/qt-5/qabstractsocket.html#waitForBytesWritten for reference._